1. Field of the Invention
The present invention relates to the field of semiconductor fabrication and, more particularly, to a mask for use in a photolithography process employed during semiconductor fabrication.
2. State of the Art
Photolithography is commonly used during the fabrication of integrated circuits on semiconductor wafers and other bulk substrates comprising a layer of semiconductor material. During photolithography, a form of radiant energy is passed through a radiation-patterning tool onto a radiation-sensitive material, commonly referred to as photoresist, which is placed upon a surface of a semiconductor wafer. The radiation-patterning tool is commonly known as a photomask or reticle. The term “photomask” is used to reference a structure that performs a function of masking or defining a pattern over an entire semiconductor wafer while the term “reticle” is used to reference a structure that functions to define a pattern over a portion of a wafer.
Radiation-patterning tools contain light-restrictive regions and light-transmissive regions. Light-restrictive regions may be, for example, opaque or partially light transmissive. The light-transmissive regions or portions of a radiation-patterning tool, in conjunction with the light-restrictive regions, cooperatively facilitate the formation of a desired pattern on a semiconductor wafer. For the formation of patterns on a semiconductor wafer, the wafer is coated with a layer of radiation-sensitive material (e.g., photoresist). Subsequently, radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers onto the photoresist a pattern defined by the radiation-patterning tool. Using a form of a photographic process, the photoresist is then developed to remove either the portions exposed to the radiant energy in the case of a “positive” photoresist or the unexposed portions when a “negative” photoresist is utilized. The residual photoresist pattern thereafter serves as a mask for a subsequent semiconductor fabrication process.
With advances in semiconductor integrated circuit processes, the dimensions associated with integrated circuit device features have decreased. Furthermore, the demand for smaller and faster-performing semiconductor devices requires increasing precision and accuracy in photolithographic processes.
FIG. 1 illustrates a flow chart of a conventional process used for creating a pattern for a radiation-patterning tool. Initially, a preliminary design is created and verified 10 with the desired pattern identified to form the desired pattern on the target photoresist. Subsequently, profiles are developed for the radiation-patterning tool to roughly produce the desired pattern when radiation is passed through the radiation-patterning tool. The profiles or elements form a rough correspondence to the desired pattern as the profiles or elements initially disregard the effects of interference of radiation passing through the radiation-patterning tool.
Following creation and verification of the design, optical proximity correction 12 accounts for various interference factors that influence radiation passing through the radiation-patterning tool. Such interference factors may include constructive and destructive interference effects resulting as the radiation wavelength approximates the dimensions of portions of the profiles or elements of the radiation-patterning tool. Optical proximity correction modifies the profile or element dimensions to shapes such that a resultant patterned photoresist more closely approximates the desired pattern. The processes of designing, verifying and optically correcting a design are typically accomplished primarily through the use of software, such as is available from Synopsys Corporation of Mountain View, Calif.
As a result of the optical proximity correction process 12, a data set that corresponds to a pattern capable of generation by a radiation-patterning tool is typically generated. The data set is subsequently “taped out” to a radiation-patterning tool through the use of, for example, laser writing and/or electron-beam writing methodologies. Following the formation of the pattern on the radiation-patterning tool, the tool is capable of being utilized for semiconductor fabrication.
FIG. 2 illustrates an exemplary apparatus 14 in which a radiation-patterning tool is utilized for a patterning process. Apparatus 14 comprises a radiation source 16 that generates radiation 18 and further includes a radiation-patterning tool 20 through which radiation 18 is passed. A semiconductor wafer or substrate 22 includes a radiation-sensitive layer 24 thereon. As illustrated, radiation 18 passes through radiation-patterning tool 20 and impacts radiation-sensitive layer 24 to form a pattern. This process of forming a pattern on a radiation-sensitive material with a radiation-patterning tool is commonly referred to as a printing process.
A radiation-patterning tool 20 typically includes an obscuring material that may either be an opaque (e.g., chrome) or a semi-opaque material placed over a transparent material (e.g., glass). Radiation-patterning tool 20 is illustrated in FIG. 2 as having a front side 28 for forming features or windows and an opposing back side 26. Some radiation-patterning tools further utilize both the front side and back side for the formation of windows.
Radiation-patterning tool 20 typically has a pattern with dimensions on the order of, or smaller than, the wavelength of radiation passing through the radiation-patterning tool. Therefore, interference effects may occur when radiation passes through the radiation-patterning tool and exits onto the radiation-sensitive material. Accordingly, the pattern of the radiation-patterning tool must be modified to compensate for such interference effects. FIG. 3 illustrates an exemplary pattern 30 desired to be formed on the radiation-sensitive material by subsequent semiconductor processes. Due to the interference effects, pattern 30 cannot be directly utilized but must undergo the optical proximity correction 12 of FIG. 1. Pattern 32 illustrates a corrected pattern that accommodates the interference effects resulting from near-wavelength dimension patterns.
FIG. 4 illustrates a radiation-patterning tool 34 and further illustrates elements utilized to create the targeted or printed images. In the exemplary printing process, a radiation-sensitive material 38 is illustrated as having formed therein a plurality of repeating patterns 40, illustrated as circular in dimension, which may be used, for example, in the formation of contact openings. One of the patterns 40 is illustrated as being centered around a location 42 while another one of the repeating patterns is illustrated as being centered around a location 44. Still referring to FIG. 4, radiation-patterning tool 34 includes a plurality of repeating elements 36 that are in a one-to-one correspondence with patterns 40 formed on the radiation-sensitive material 38. As shown, each of elements 36 is approximately square in shape that, when passing radiation therethrough, forms the circular patterns 40 on radiation-sensitive material 38. Elements 36 on the radiation-patterning tool 34 may be either more transparent to radiation than surrounding regions or less transparent, depending on whether the radiation-sensitive material 38 is implemented as a positive or negative photoresist material. When elements 36 are more transmissive to radiation than surrounding regions, elements 36 effectively act as windows that allow radiation to pass through onto the radiation-sensitive material 38.
The printed patterns 40 correspond to regions where light has passed through elements 36 of the radiation-patterning tool 34. As described above, when elements 36 exhibit dimensions approximating the wavelength of the radiation, interference effects may occur. FIG. 4 illustrates interference effects in the form of sidelobes 46 extending around each of the patterns 40. Exposure of radiation-sensitive material 38 to a single sidelobe 46 generally does not result in a printed feature within the radiation-sensitive material 38. However, when two or more sidelobes 46 overlap, it is possible to form a printed feature. Regions 48 and 50 illustrate locations where, specifically, four sidelobes converge, and accordingly illustrate locations where printed features may be undesirably manifested.
FIGS. 5-7 further describe sidelobe convergence in additional detail with respect to the interference effects of radiation having a wavelength that is on the order of the dimensions of the desired pattern. In FIG. 5, the electric field strength of radiation passing through an element 36 (FIG. 4) is diagrammatically illustrated for forming a pattern 40 (FIG. 4) centered around a location 42. As illustrated, a large, positive field strength occurs at location 42, which creates undesirable sidelobes 46. A large, positive field strength centered around location 42 may be referred to as a primary lobe while the lobes or concentrations of energy centered away from the primary lobe are referred to as sidelobes 46.
The exposure of radiation-sensitive material 38 is proportional to the intensity of the radiation rather than the field strength, as the intensity is a function of the square of the field strength. FIG. 6 illustrates the intensity of the radiation utilized to pattern the feature centered around location 42. Accordingly, since the intensity is the square of the field strength, the sidelobes 46 have a positive value as does the primary lobe centered around location 42. Therefore, if the magnitudes of the sidelobes 46 are sufficient, the sidelobes can induce printed features in the radiation-sensitive material 38. FIG. 7 illustrates the additive effect of a sidelobe formed from radiation centered around location 42 and a sidelobe formed from radiation centered around location 44. The radiation formed around location 44 is consistent in magnitude with that formed around location 42. Accordingly, since the adjacent sidelobes overlap, the two sidelobes combine to form a resultant overlapping lobe 52. Lobe 52 results from the combination of overlapping sidelobes of adjacent patterns of radiation. Such an overlapping combination, as illustrated in FIG. 7, can be extended to combinations of 3, 4 or more proximate patterns of radiation. Accordingly, the energy combination of adjacent overlapping sidelobes can grow significantly in intensity relative to the main lobes, which can eventually result in a generated printed feature.
Identification of locations where sidelobes may combine to form a printed feature has been undertaken and, when such locations are identified, a radiation-patterning tool can be modified to prevent the undesired combination of sidelobes. FIG. 8 illustrates an arrangement 54 that identifies design elements illustrated as design features 56 corresponding to elements 36 (FIG. 4) of a radiation-patterning tool. One prior approach for identifying locations includes a mathematical calculation performed on the spatial characteristics of design features 56 to create a common region 58 extending between design features 56. From the region 58, a sidelobe inhibitor 60 may be calculated. The sidelobe inhibitor is utilized to prevent formation of an undesired printed feature from occurring at the location where sidelobes converge.
FIG. 9 illustrates a portion of a prior art radiation-patterning tool 34 similar to the tool described with reference to FIG. 4, except that FIG. 9 further illustrates a sidelobe inhibitor 62 for preventing sidelobes of radiation from combining when passing through elements 36. Sidelobe inhibitor 62 typically includes dimensions of approximately one-half of the wavelength of the radiation passed through radiation-patterning tool 34. Sidelobe inhibitor 62 may be formed, for example, by etching onto an opaque material associated with radiation-patterning tool 34 to form a region where radiation will be in phase with the main energy lobe and thus out of phase relative to other portions of the sidelobe radiation. Such a combination is typically known as destructive interference, which results in a cancellation of a significant amount of intensity from the combined sidelobes.
Accordingly, there is a need and desire to minimize and even eliminate sidelobe effects on a radiation-patterning process.